Purine and pyrimidine nucleosides and bases, the essential building blocks of nucleic acids, occur widely throughout the animal kingdom and underlie a number of critical functions including energy transduction, metabolism and cell signaling. One endogenous purine nucleoside, adenosine (ADO), plays an important role in a number of biochemical processes including energy transfer.
In the nervous system, ADO acts as a non-classical inhibitory neurotransmitter (Pedata, et al. (1983) Neuropharmacol. 22:609-614; Jackisch, et al. (1984) Naunyn Schmiedebergs Arch. Pharmacol. 327:319-25) and neuromodulator (Phillis & Wu (1981) Prog. Neurobiol. 16:187-239; Snyder (1985) Annu. Rev. Neurosci. 8:103-124). Alterations in adenosine or its signaling have been linked to a number of neurological disorders including epilepsy (Boison (2012) Glia 60:1234-43), Parkinson's disease (Wardas (2002) Pol. J. Pharmacol. 54:313-26; Schwarzschild, et al. (2006) Trends Neurosci. 29:647-54), schizophrenia (Boison, et al. (2012) Neuropharmacology 62:1527-43), panic disorder and anxiety (Hohoff, et al. (2010) J. Psychiatr. Res. 44:930-7), as well as drug abuse (Ferre, et al. (2007) Prog. Neurobiol. 83:332-47). Alterations in ADO have also been linked to changes in a sleep and arousal (Porkka-Heiskanen & Kalinchuk (2011) Sleep Med. Rev. 15:123-35) as well as cognition and memory (Wei, et al. (2011) Learn. Mem. 18:459-74; Fredholm, et al. (2005) Int. Rev. Neurobiol. 63:191-270).
With the brain, extracellular ADO concentrations have been reported to be in the 30-400 nM range (Fredholm, et al. (1999) Pharmacol. Rev. 51:83-133; Zetterström, et al. (1982) Neurosci. Lett. 29:111-115). However, in response to cellular damage (e.g., seizure or ischemia), these concentrations can quickly elevate (Zetterström, et al. (1982) supra; Berman, et al. (2000) Brain Res. 872:44-53), in some cases 7.5-31-fold (During & Spencer (1992) Ann. Neurol. 32:618-24), suggesting that ADO, in addition to signaling, also can have a neuroprotective function.
Adenosine functions by binding to and signaling through four known receptor subtypes (A1, A2A, A2B, and A3) (Gomes, et al. (2011) Biochim. Biophys. Acta 1808:1380-99; Sebastiao & Ribeiro (2009) Handb. Exp. Pharmacol. 471-534). One of the best-known compounds that acts via ADO signaling, and in particular by bind to the A2A receptor, is caffeine. This drug's stimulatory effects are primarily (although not entirely) credited to its inhibition of ADO via competitive inhibition of these receptors (Lazarus, et al. (2011) J. Neurosci. 31:10067-75), effectively blocking adenosine signaling. The subsequent reduction in ADO signaling leads to increased activity of other neurotransmitters including acetylcholine (Jin & Fredholm (1997) Naunyn Schmiedebergs Arch. Pharmacol. 355:48-56), noradrenaline (Allgaier, et al. (1991) Naunyn Schmiedebergs Arch. Pharmacol. 344:187-92), GABA (Kirk & Richardson (1995) J. Neurochem. 64:2801-9), dopamine (Ferre, et al. (1991) Proc. Natl. Acad. Sci. USA 88:7238-41) and glutamate (Rodrigues, et al. (2005) J. Neurochem. 92:433-41).
Conventionally, the ability to directly measure adenosine and its metabolites has been difficult, and has generally been carried out by proxy. For example, aptamer-based approaches have been described (Zhou & Zhao (2012) Analyst 137:4262-6; Liu & Yan (2012) Biosens. Bioelectron. 36:135-41; Li, et al. (2012) Anal. Chem. 84:2837-2842; Huang & Liu (2010) Anal. Chem. 82:4020-26; Yan, et al. (2009) Talanta 79:383-87; Kim, et al. (2009) Chem. Commun. 31:4747-9), as have label-based (Huang, et al. (2011) Biosens. Bioelectron. 29:178-83) and enzyme-based, e.g., luciferase (Burgos, et al. (2012) Anal. Chem. 84:3593-8; Simpson, et al. (2008) Lett. Appl. Microbiol. 11:208-10)or S-adenosylhomocysteine hydrolase (WO 1999/034210; U.S. Pat. No. 6,395,256) approaches.
More direct approaches have also been suggested. For example, a combined high pressure liquid chromatography (HPLC) separation and electrochemical detection method has been described for identifying adenosine and guanosine (Henderson & Griffin (1984) J. Chromatgraph. A 298:231-42). However, clean-up procedures were suggested to remove interfering material in biological samples. Similarly, a combined reversed-phase HPLC, gradient elution and electrochemical and UV detection method has been described for detecting adenosine, guanosine, inosine, guanine, hypoxanthine, xanthine and urate in mouse brain and serum, as well as in post-mortem human brain (Burdett, et al. (2012) Biomed. Chromatograph. Doi:10.1002/bmc.2760). Moreover, simultaneous detection of dopamine and adenosine has been described using a boron-doped diamond working electrode in combination with HPLC (Birbeck & Mathews (2012) Pittcon 2012, Abstract 1370-3; Birbeck & Mathews (2013) Anal. Chem. 85:7898-404).